Underwater electroacoustic transducer which resists intense pressure



;, Nov. 10, 1970 F. MASSA. JR 3,539,930

' UNDERWATER ELECTROACOUSTIC TRANSDUCER WHICH RESISTS INTENSE PRESSURE Filed Nov. 29, 1968 lNVEN r09.

F/PA A/K M4554 JR United States Patent 3,539,980 UNDERWATER ELECTROACOUSTIC TRANS- DUCER WHICH RESISTS INTENSE PRESSURE Frank Massa, J13, Cohasset, Mass., assignor to Massa Division, Dynamics Corporation of America, Hmgbam,

Mass.

Filed Nov. 29, 1968, Ser. No. 779,938

Int. Cl. H04r 1/02 U.S. Cl. 3408 13 Claims ABSTRACT OF THE DISCLOSURE This invention relates to underwater transducers, and more particularly to transducers which must withstand intense ambient pressure.

As used herein, the term intense pressure implies pressures in the order of 3000-3500 (or more) pounds per square inch. However, the invention is not limited to these or any other specific values. These intense pressures might occur responsive to underwater explosions. They also occur in very deep water, and perhaps elsewhere. Obviously, the converse is also truethe transducer should be able to operate equally well at the other end of the scale, where ambient pressure is negligible, as in gases or shallow liquids. Otherwise, the transducer would not function equally well over the entire range through which it passes, as when it is lowered from the surface to deep water.

Transducers constructed according to the teachings of this invention find many uses, one of which may be on a submarine. When so used, the transducer may be partly recessed within a hull or deck so that its only exposed part is a plate mounted flush on the deck. If so, the transducer must withstand the abuse which occurs when people walk on it or drag heavy objects over it. Still other design considerations will readily occur to those skilled in the art.

The preferred transducer design utilizes a vibratile plate piston driven by a stack of piezoelectric elements. These elements mechanicaly vibrate the plate to generate an acoustic power which is transferred to any gas or liquid surrounding the transducer. One example of a transducer of this type may be found in U.S. Patent No. 3,328,751.

Accordingly, an object of this invention is to provide new and improved underwater transducers which are able to withstand intense pressures. In this connection, an object is to provide such electroacoustic transducers having improved shock resistance characteristics irrespective of the surrounding pressures. Here an object is to provide transducers having means for equalizing pressures inside and outside the transducer.

Another object of this invention is to provide rugged transducers which can withstand abuse resulting from people walking on it and dragging heavy objects over it.

In keeping with an aspect of the invention, these and other objects are accomplished by a piezoceramic transducer assembly floating in a silicone rubber cradle. A rigid steel housing, filled with a sound transmitting oil, surrounds the rubber cradle. The oil fills all voids within the transducer and housing. A rubber or other flexible member exposed to surrounding ambient pressure expands or contracts to increase or decrease the pressure of the oil within the transducer and thereby equalize the pressure inside the housing with the pressure outside the housing. Since there is a zero pressure differential between the inside and outside of the housing, the transducer does not experience intensive crushing forces.

The nature of a preferred embodiment incorporating the features and advantages of the invention will be understood best from a study of the following description when taken in connection with the attached drawings, in which:

FIG. 1 is a cross-sectional view of a first embodiment of the invention featuring a flexible window for transmitting vibrations from the transducer into the associated medium and for equalizing pressures within and without the housing; and

FIG. 2 is a cross-sectional view of a second embodiment of the invention featuring a steel window for transmitting vibrations and a flexible sleeve for equalizing pressure.

Both of the embodiments of the invention include essentlally the same structure, which may be generally described as a transducer 20 and a cradle 21 of elastic material inside a rugged protective housing 23. The transducer 20 includes a stack of piezoceramic elements 24-27 positioned between the back of a. vibratile plate 28 and the front of an inertial mass element 29 which are held together by a stress bolt 30.

The piezoelectric material 24-27 may be a stack of coaxially positioned ceramic rings made of any known material, such as a polarized lead zirconate titanate. Suitable cement may be used to help hold the piezoelectric assembly together. The ceramic rings are separated by electrodes joining like polarities of the adjacent piezoelectric material. The electrodes of one polarity (say positive) are connected together by a first wire, as at 31. The electrodes of the other polarity (negative in this case) are connected together by a second wire, as at 32. These wires 3.1, 32 are connected to a cable 33 via insulated terminals 34, 35.

To enable an easy replacement of the electrical cable 33, a massive end plate 36 includes a cavity 37 where connections may be made, or changed, between the terminals 34 and 35. This cavity 37 is closed by a cover plate 38 which may be bolted in place. An O-ring 39 provides a hermetic seal between plates 36, 38. A waterproof molded rubber section 40 seals the cable 33 to the cover plate 38 and encloses the terminals 35.

The operation of a transducer of this type is well known, being explained in U.S. patent No. 3,328,751 and elsewhere. An alternating current source (not shown) is connected to the other end of the electrical cable 33. When power from this A.C. source is applied, the stack of piezoelectric ceramic rings 24-27 experiences corresponding mechanical excursions, and the plate piston 28 vibrates. As a result, sonic energy is radiated outwardly from the end of the transducer assembly.

According to the invention, the transducer assembly floats in a cradle of silicone rubber. That cradle is protected by a strong and rugged steel. housing or case which is spaced away from and surrounds the peripheral portions of the vibratile plate 28 and the inertial mass element 29. In greater detail, the housing 23 is made of a strong, heavy walled steel tube 41 welded to the massive end plate 36 which surrounds the back of the inertial mass element 29. Then the assembled transducer 20 is slipped into the open end of the housing 23. Thereafter, the housing may he stood on its end plate 36, and an uncured liquid silicone rubber compound 42 is poured into the space between the housing 23 and the transducer 20, after which the rubber may be cured in any kn wn manner. Thus, the peripheral and back portions of the inertial mass element 29 are flexibly bonded to the steel housing 23 and back plate 36', via a resilient rubber cradle mount 21. To help anchor the inertial mass 29 and lock it in place, it has an annular groove 43 which fills with the liquid rubber.

An alternative construction would be to pre-form a rubber cap, as shown at 42, which is snapped over the inertial mass element 29 before the transducer is slipped into the steel housing 23. Cement may be used to seal the rubber cap 42 to the mass 29 and the steel housing 23.

After the inertial mass 29 is anchored in place, a resilient annular collar 45 is fitted into the peripheral space between the open end of the housing 23 and the vibratile piston plate 28. Preferably, this collar 45 is a material having a very low coeflicient of friction, such as a silicone rubber. There should not be any appreciable loss of mechanical energy at the interface between the plate piston 28 and the resilient collar 45.

After the transducer is completely nested Within its rubber cradle 21 (comprising caps 42, 45), a plate 46 is attached, in front of the vibratile plate, to the housing 23 by means of bolts 47, 48. The plate 46 rests against the end portion of the resilient collar 45 in order to provide a positive axial location of the transducer 20. In order to provide a hermetic seal, an O-ring 49 is compressed between the plate 46 and the end of the steel tube 41. The plate 46 includes a sound transparent window or a central opening closed by a rubber or other flexible diaphragm 51 molded therein.

Means are provided for maintaining an equilibrium of pressure between the inside and the outside of the transducer assembly. To do this, all voids or cavities inside the transducer are interconnected by communicating passageways. For example, the collar 45 is pierced by a passageway 53, and the vibratile plate piston 28 is pierced by a passageway 54. These passageways and the associated cavities are, in turn, accessible to the outside of housing 23 via an opening which is sealed by a tapered, threaded plug 55.

To complete the pressure equalizing mechanism, the plug 55 is removed, and the housing 23 is evacuated. Then, a sound transmitting fluid is injected, under a slight positive pressure, into the opening at 55. The sound transmitting fluid flows into the housing, through all communicating passageways, and into all trapped enclosures, cavities, or voids. The transducer is completely filled, and the diaphragm 51 has a slight bulge. Then, the plug 55 is turned into place to seal the housing and retain the fluid. While any of many suitable sound transmitting fluids may be used, I prefer to use a silicone oil or castor oil.

The embodiment of FIG. 2 is essentially the same as that of FIG. 1 except that the plate 46 is replaced by a solid steel plate 46a. Instead of a window filled by a molded resilient diaphragm 51, the plate 46a has an undercut region 59. Owing to the differential in plate thickness, the region 59 functions efliciently as a sound transmitting window. Nevertheless, the solid, unbroken outer surface of the steel plate 46a is not damaged by the mechanical abuses which may occur, as when people walk on it or drag heavy objects over it.

To provide for the introduction of a pressure equalizing fluid, the plate 46a is provided with a tapped opening plug 60. A resilient sleeve 61 (which could be rubber) surrounds the steel walled tube 41a. Sleeve 61 is clamped in place on tube 41a by means of metal bands 62, 63 at either end. A small orifice 64 provides a pressure communicating passageway between the inside of the housing Wall 41a and the inside of the resilient sleeve 61.

The plug 60 is removed, and the transducer housing is evacuated. Thereafter, a sound transmitting fluid is introduced under positive pressure. All entrapped spaces are filled until the resilient sleeve 61 bulges. Then, the plug 60 is replaced.

This way, any ambient pressure change outside the transducer is translated through the resilient sleeve 61 and the passageway 64 to the inside of the transducer. Hence, there will never be any substantial pressure differential between the inside and the outside of the transducer. This essentially zero pressure differential prevents damage responsive to intense ambient pressures acting upon the transducer.

Still other embodiments will readily occur to those who are skilled in the art. Hence, the appended claims are to be construed broadly enough to cover all equivalents falling within the true scope and spirit of the invention.

I claim:

1. An electroacoustic transducer comprising a vibratile plate member having a front, back, and peripheral portion, an inertial mass member having a front, back, and peripheral portion, electromechanical oscillatory force generating means operatively connected between the back of said vibratile plate member and the front of said inertial member, a rigid housing structure having an inner peripheral wall portion spaced away from and surrounding said peripheral portions of said vibratile plate member and said inertial mass member, the back of said housing being spaced away from and enclosing the back of said inertial member, and resilient cradle means comprising a compliant material filling all of the clearance space between said housing structure and the peripheral portion of said vibratile plate member and the peripheral and back portions of the inertial member.

2. The transducer of claim 1 and sound conducting liquid means completely filling all air space within the rigid housing.

3. The transducer of claim 2 wherein the end of said housing structure in front of said plate is open, and waterproof sealing means including a sound transparent window for closing said open end.

4. The transducer of claim 3 wherein said sound transparent window consists of a flexible material, and sound conducting liquid means filling all air space within the assembled structure with a slightly positive pressure whereby the flexible window bulges outwardly as a result of the liquid pressure.

5. The transducer of claim 1 wherein said compliant cradle material is a silicone rubber.

6. An electroacoustic transducer comprising a vibratile plate member having a peripheral portion, an inertial mass member having a peripheral portion, electromechanical oscillatory force generating means operatively connected between said vibratile plate member and said inertial member, a rigid housing structure having an inner peripheral wall portion spaced away from and surrounding said peripheral portions of said vibratile plate member and said inertial mass member, resilient cradle means comprising a compliant material filling most of the clearance space between said housing structure and the peripheral portions of said vibratile plate member and inertial member, said rigid housing structure being closed at one end, said inertial member being located near and spaced away from said closed end, said compliant material filling most of the space between said inertial member and the closed end of said housing structure, the other end of said housing structure being open, waterproof sealing means including a sound transparent window for closing said open end, wherein said sound transparent window is a rigid plate.

7. The transducer of claim 6 and means comprising a flexible walled enclosure surrounding a portion of said rigid housing structure, an orifice through the wall of said housing structure communicating between the inner space inside said rigid housing structure and the space within said flexible walled enclosure surrounding said housing structure, and sound conducting fluid means filling all air space inside said flexible walled enclosure and said rigid housing, said fluid communicating through said orifice and completely filling said flexible walled enclosure.

8. An intense pressure resistant transducer assembly comprising transducer means floating in a resilient cradle surrounded and protected by a rigid housing, sound transmitting fluid filling all voids within said rigid housing, and means for transmitting ambient pressure surrounding said assembly to said fluid, thereby equalizing the pressure inside the housing with the pressure outside the housing.

9. An intense pressure resistant transducer assembly comprising transducer means floating in a resilient cradle surrounded and protected by a rigid housing, sound transmitting fluid filling all voids within said rigid housing, means for transmitting ambient pressure surrounding said assembly to said fluid, thereby equalizing the pressure inside the housing with the pressure outside the housing, wherein said transducer means comprises, in series, a vibratile plate piston, a piezoelectric material structure and an inertial mass element held together by a stress bolt.

10. The assembly of claim 9 wherein said cradle comprises a resilient means interconnecting said plate and said housing and interconnecting said mass element and said housing.

11. The assembly of claim 10 wherein said ambient pressure transmitting means comprises a sound transmitting window adjacent said vibratile plate piston.

12. The assembly of claim 10 wherein said ambient pressure transmitting means comprises a resilient sleeve at least partially surrounding said rigid housing.

13. The assembly of claim 12 wherein said rigid housing comprises an undercut steel plate adjacent said plate piston for transmitting sonic energy outwardly from said assembly.

References Cited UNITED STATES PATENTS 2,983,901 5/1961 Paslay et al. 340-10 3,199,071 8/1965 Massa 340-10 3,230,503 1/1966 Elliot et al. 3401O RODNEY D. BENNETT, Primary Examiner B. L. RIBANDO, Assistant Examiner US. Cl. X.R. 34010 

